SPEEDOMETER AND TACHOMETER
The speedometer used on this vehicle indicates miles
and kilometers per hour. Distance traveled is recorded
(in miles*) on an odometer. Speedometer is driven by a
semi-rigid cable connected to a set of gears in vehicle
transmission. Specified gears match the particular truck
model and take into consideration such other factors as
tire size and rear axle ratio. The semi-rigid cable which
connects transmission driven gear to the speedometer
head consists of an outer case and an inner core.
Odometer records up to 999,999 miles* and returns to
zero. School bus models record 99,999.9 miles.* (*Total
metric speedometers record kilometers.)
Should the need arise for changing a truck axle ratio or
tire size, a corresponding change must also be made to
the speedometer drive. This is accomplished by
changing the speedometer adapter and in some
instances the driven gear in the truck transmission.
Information for finding the correct adapter and driven
gear can be found in the Speedometer Section of the
An electrical or mechanical tachometer (not standard for
all vehicles) is also located in the instrument cluster to
record engine RPM (Revolutions Per Minute). This
instrument enables driver to keep engine speed within an
efficient operating range.
The mechanical tachometer is driven by a flexible shaft
connected to an adapter or driving unit located on engine
(distributor shaft for gasoline engines or air compressor
injection pump, auxiliary shaft, etc. for diesel engines).
The electric tachometer reads the same as the
mechanical unit but receives electrical impulses by wire
instead of rotating cable. These impulses are received
from a sender unit mounted on the engine and using the
mechanical tachometer drive take off.
SPEEDOMETER AND TACHOMETER CABLES
A semi-rigid type cable is used for both speedometer and
mechanical tachometer on S-Series trucks.
This cable consists of two segments:
An outer case assembly with nut and ferrule at
lower end and a quick-disconnect ferrule at
A wire-wound flexible inner core assembly with
squared drive at upper end and floating tip
(Figure 14) at lower end.
The floating tip which is detachable from cable allows
core to float and thereby minimize thrust on speedometer
or tachometer head. (Speedometer cable assemblies
are sometimes made in two sections for assembly
Figure 15 - Semi - Rigid Cable Outer Case
The cable outer case is formed from an inner nylon liner,
an intermediate ply of wire braid and an outer
polypropylene or polyethylene case (Figure 15).
Figure 14 Semi-Rigid Cable End Details
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